Power tongs are well known in the field of drilling and servicing oil and gas wells. Drill pipe and production tubing are typically provided in the form of round steel pipe (commonly referred to as tubulars) with threaded ends for connecting tubulars into a drill string or a production string, depending on the operation being conducted. The term “make-up” is commonly used to refer to the process of connecting tubulars to each other (i.e., “making up” a threaded connection), and the term “break-out” refers to the process of disconnecting tubulars (i.e., “breaking out” a threaded connection). Well drilling and well servicing involve both make-up and break-out functions, for a variety of purposes well known in the field.
Make-up or break-out of a threaded joint requires that the tubular on each side of the joint be firmly gripped so that the tubulars can be contra-rotated relative to each other, either clockwise or counterclockwise, to make up or break out the joint as desired. This gripping function is commonly carried out using a power tong on each of the tubulars. Power tongs typically have either sliding jaw assemblies or pivoting jaw assemblies, and the present invention is particularly referable to the pivoting-jaw type. There are numerous known types or models of pivoting-jaw power tongs, but they generally have the common features of a partial-circle (or “C-shaped”) rotary gear, the internal surface of which defines a camming surface, and a jaw assembly disposed inside the rotary gear and having two or more cam followers that ride against the camming surface of the rotary gear. The cam followers may be in the form of rollers which turn around suitable axles or pivot pins, or they may be formed integrally with the jaws. The jaw assembly has several (typically four) dies—i.e., elements which are toothed or otherwise adapted for grippingly engaging the circumferential outer surface of a tubular member by effectively biting into the steel surface of the tubular when forced against the tubular. The geometry of the camming surface is adapted such that when the rotary gear is rotated around the jaw assembly, either clockwise or counterclockwise away from a neutral position, the dies are urged into gripping contact with the outer surface of the tubular. When the tubulars on each side of the joint have been thus engaged by respective power tongs, the tongs may be rotated relative to each other in the desired mode, thus making up or breaking out the joint as desired.
One example of a prior art pivoting-jaw power tong is disclosed in Canadian Patent No. 1,125,737 issued on Jun. 15, 1982 to Farr et al. (and corresponding to U.S. Pat. No. 4,350,062). As with typical pivoting-jaw power tongs, the camming surface of the Farr device includes a pair of opposed recesses (or neutral zones) such that when the rotary gear is rotated to a neutral position wherein each cam follower has moved into one of the recesses, the jaws spread apart so as to allow the jaws to receive a tubular. On either side of each recess, the camming surface has a circularly-curved primary camming surface. These primary camming surfaces are configured such that when the rotary gear is rotated in either direction away from the neutral position, each jaw is rotated inward. As rotation of the rotary gear increases, the jaws close on the tubular, causing the dies to bite into the tubular. When the tubular is firmly gripped by the dies, the tubular can be rotated so as to be connected to or disconnected from (as the case may be) another tubular.
The primary camming surfaces of the Farr device have different curvature radii on either side of each recess, and this is considered to have certain advantages over typical prior art pivoting-jaw power tongs in which the same curvature radius is used for all of the primary camming surfaces. However, it has been observed that power tongs with circular primary camming surfaces are prone to reduced effectiveness as the dies, pins, rollers, and/or camming surfaces become worn. In such circumstances, the force with which the dies are urged against the surface of the tubular is not uniform, so the dies grip the tubular with variable effectiveness, and in the worst case a die may grip the tubular with little or no effectiveness at all. In such cases only three of the dies will be effectively gripping the tubular instead of four, and this condition tends to causing warping and/or marking of the tubular. This tends to be a particular problem for the dies located near the pivot points of the jaws.
For the foregoing reasons, there is a need for improved pivoting-jaw power tongs having enhanced capability for reliable and effective gripping engagement of tubular members in cases where dies, pins, rollers, and/or camming surfaces are worn. The present invention is directed to this need.